NESTLING AMERICAN ROBINS COMPETE WITH SIBLINGS BY BEGGING

The evolution of intense begging by dependent nestling birds has recently been the subject of several theoretical papers. The interesting problem here is that nestlings should be able to communicate their nutritional status to parents in ways that are less costly energetically and less likely to attract predators. Thus, conspicuous begging behaviour is thought to have evolved as a result of either competition among nestmates or the manipulation of their parents to provide more food than would otherwise be favoured by selection. We studied sibling competition for parental feedings in the American robin (Turdus migratorius). We demonstrate that the probability that an individual nestling received food was related to several indices of begging. When we experimentally prevented parents from feeding part of their brood, both the intensity of begging and the number of feedings subsequently received by food-deprived nestlings increased. Furthermore, the begging intensity of those nestlings that were not food-deprived also increased in response to the begging of their hungrier siblings.